Transfer Pump DIY Success for $40
I just wanted to report that I replaced the plastic bodied (late style) fuel transfer pump on my 535i with the pump intended for a 1975 carbureted Chevy Vega.
For anyone who isn't familiar, the late model e28's have a plastic bodied pump that includes the mounting bracket. On the earlier models there is a mounting bracket that can be transferred over to the new pump.
The parts I used were Airtex E2485 and FS100. I replaced the fuel strainer as the original didn't fit well and I didn't want to take any chances with it getting brittle over time. The transfer pump is low pressure, hence why a pump intended for a carbed engine works well.
I also used a hose clamp to hold the pump to the return line.
I didn't take any pictures during the job, but it couldn't have been easier.
A quick walkthrough though in case anyone's interested:
1) The pump access is through the trunk. Remove the bottom liner and the three philips screws covering the access cover. Clean the whole area under this to keep anything from falling into the tank.
2) Remove the sender unit by removing the four 8mm nuts and lifting it up. The sender has to be removed first in order to pull the pump assembly up.
3) Remove the 8mm bolts holding the pump assembly to the tank and two fuel hoses.
4) Once the assembly is removed, the old pump can be pulled. The new one goes on easily. Since the bracket was built into the old pump it can't be reused. Instead I used one of the grommets from the new Airtex pump around the metal clip on the return line as vibration isolation and a short piece of fuel line around the return line itself. I used a large hose clamp around the two so with two pieces of fuel resistant rubber as insulation and being held tightly together the pump is held solidly without risk of metal to metal rubbing.
5) I had to run a new power wire as the old wasn't quite long enough. Simple enough to solder on. The old pump was grounded to the plate assembly (the second wire on the plug is electrically attached to it) so I ran another wire from the pump's ground lug to the screw that used to attach the pump to the bracket. If you see it in person it makes perfect sense.
That said, my pump worked fine. I found the actual source of the problem as soon as I pulled my old pump out. Just like on the Airtex pump there are two lugs for power. The hot is wired to the connector but the ground travels along a thin copper strap to the pump mounting point. This thin strap on mine was fractured in the middle. Looking at the fracture ends it was a brittle failure, likely induced from vibration. It's not surprising as the strap is not very flexible and is forced to undergo small deflections as the pump motor vibrates. Instead of patching up the wire for it to fail again and since I already had the parts I went through with the replacement. But I'd have to wonder if this isn't a more common issue.
My car idles much better now, even with an exhaust leak at the manifold that will be fixed this weekend.